The following shell script finds duplicate (2 or more identical) files and outputs a new shell script containing commented-out rm statements for deleting them.
You then have to edit the file to select which files to keep – the script can’t safely do it automatically!

OUTF=rem-duplicates.sh;
echo "#! /bin/sh" > $OUTF;
find "$@" -type f -print0 |
  xargs -0 -n1 md5sum |
    sort --key=1,32 | uniq -w 32 -d --all-repeated=separate |
    sed -r 's/^[0-9a-f]*( )*//;s/([^a-zA-Z0-9./_-])/\\\1/g;s/(.+)/#rm \1/' >> $OUTF;
chmod a+x $OUTF; ls -l $OUTF

Example output (rem-duplicates.sh)

#! /bin/sh
#rm ./gdc2001/113-1303_IMG.JPG
#rm ./reppulilta/gdc2001/113-1303_IMG.JPG

#rm ./lissabon/01-01-2001/108-0883_IMG.JPG
#rm ./kuvat\ reppulilta/lissabon/01-01-2001/108-0883_IMG.JPG

#rm ./gdc2001/113-1328_IMG.JPG
#rm ./kuvat\ reppulilta/gdc2001/113-1328_IMG.JPG

Explanation

1. write output script header
2. list all files recursively under current directory
3. escape all the potentially dangerous characters with xargs
4. calculate MD5 sums
5. find duplicate sums
6. strip off MD5 sums and leave only file names
7. escape strange characters from the filenames
8. write out commented-out delete commands
9. make the output script writable and ls -l it